The present invention relates generally to wireless communication systems, and more particularly, to a method and an apparatus for controlling power during a dispatch group call.
A wireless communication system is a complex network of systems and elements. Typical elements include (1) a radio link to the mobile stations (e.g., cellular telephones), which is usually provided by at least one and typically several base stations, (2) communication links between the base stations, (3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication between and to manage the operation and interaction of the base stations, (4) a call controller or switch, typically a call agent (i.e., a xe2x80x9csoftswitchxe2x80x9d), for routing calls within the system, and (5) a link to the land line or public switch telephone network (PSTN), which is usually also provided by the call agent.
Wireless communication systems typically provide point-to-point service such that a one-to-one correspondence exists between a mobile station that transmits a signal (i.e., talk) and a mobile station that receives the signal (i.e., listen). In contrast, a dispatch system such as, but not limited to, a police radio system and a taxi system, provides a one-to-many correspondence between a mobile station that transmits a signal and a number of mobile stations that receive the signal. During a dispatch group call, an entire group of mobile stations xe2x80x9clistenxe2x80x9d to the same forward link signal, and a majority of the mobile stations are in a xe2x80x9cpassivexe2x80x9d mode (i.e., listening) while only one mobile station is in an xe2x80x9cactivexe2x80x9d mode (i.e., talking) at any one time. Information such as a voice and/or data transmission from the mobile stations flow to the base station via a reverse traffic channel. For example, a user of a mobile station may activate the xe2x80x9ctalkxe2x80x9d function by selecting or pressing a xe2x80x9cpush-to-talkxe2x80x9d (PTT) button on the mobile station. Accordingly, the voice or data transmission from the mobile station of the user is communicated on a reverse traffic channel to the base station for transmission on a forward broadcast channel to the other mobile stations listening to the dispatch group call.
One aspect of designing a communication system is to maximize the capacity of the system in terms of the number of calls that can be handled simultaneously. The capacity of the system can be maximized when the transmission power of each mobile station is controlled so that each signal transmitted by the mobile stations arrives at a base station servicing those mobile stations at approximately the same power level. In contrast, when a signal transmitted by a mobile station arrives at the base station at a power level that is too low in comparison to the power level of other mobile stations serviced by the base station, the bit error rate (BER) may be too high to permit high quality communication because of interference from the other mobile stations. When a signal transmitted by the mobile station arrives at the base station at a power level that is too high in comparison to the power level of other mobile stations serviced by the base station, communication with this particular mobile station is acceptable but this high power signal acts as interference to the other mobile stations.
As in most wireless communication systems, transmission from a base station on the forward broadcast channel may be too weak in some parts of a coverage area for a mobile station to achieve high quality communication with the base station. Typically, the transmission becomes weaker as the mobile station moves farther away from the base station. Another example may be a location where the path loss of one or two neighboring base station(s) is approximately the same as the path loss of the base station in communication with the mobile station. As a result, transmission power needs to be increased to avoid poor quality in the transmission from the base station on the forward broadcast channel. In addition, interference from other neighboring base stations may require additional power to service the mobile station adequately. In contrast, the mobile station may be in a part of the coverage area where the signal-to-interference ratio (SIR) is unusually favorable for the mobile station. As a result, the base station may transmit on the forward broadcast channel using a lower than normal transmission power so that interference to other transmission in the system may be reduced. Therefore, a need exist to maximize capacity of a communication system by controlling transmission power of each mobile station within the coverage area serviced by the base station.